Transition between coalescence and bouncing of droplets on a deep liquid pool

This study focused on the bouncing of sub-millimetric droplets (below 0.7 mm) of three different fluids, distilled water, technical ethanol and 1-propanol on a deep liquid pool of the same fluids. Four different flow regimes including low-energy-collision coalescence, bouncing, high-energy-collision coalescence, and partial coalescence were observed in the experiments. These regimes were plotted in velocity-diameter diagrams, which showed that there was a diameter limit, D ≈ 0.2 mm, above which the low-energy-collision coalescence was inhibited. The contact time, in which the impinging droplets and the liquid surface interacted in the bouncing process, was studied, and the results showed the same characteristic time scale of the contact time as those of Richard et al. (in Nature 417, 2002, 811) and Thoroddsen and Takehara (in Phys. Fluids 12 (6), 2000, 1265–1267). The restitution coefficients for all fluids were investigated, and the water data agreed well with the values reported in the literature (Bach et al., J. Fluid. Mech. 518, 2004, 157–185; Jayaratne and Mason, Proc. R. Soc. Lond. A 280 (1383), 1964, 545–565). Based on stable restitution coefficients, which varied with fluids, the effects from both viscosity and surface tension were discussed. Further, a correlation (K = We · Oh^−0.58) was generalized to characterize the two transitions between coalescence (both high-energy- and low-energy-collision types) and bouncing, and a comparison with the model and data of Huang and Zhang (in Petrol Sci. 5, 2008, 62–66) showed that the generalized model characterized the coalescence-bouncing threshold well for the experimental fluids in the present study and oil with much higher viscosity.